Method of operating an ion-getter vacuum pump with gun and grid structure arranged for optimum ionization and sublimation

ABSTRACT

ELECTRONS INJECTED BETWEEN A CYLINDRICAL HOUSING AND A COAXIAL CYLINDRICAL GRID AT A PREDETERMINED ANGULAR MOMENTUM SPIRAL OVER LONG PATHS AROUND THE GRID FOR OPTIMUM IONIZATION OF GAS MOLECULES. A FIRST ELECTROSTATIC FIELD IS ESTABLISHED BETWEEN THE HOUSING AND GRID FOR ACCELERATING IONS TO THE HOUSING AND FOR ESTABLISHING AN OPTIMUM KINETIC ENERGY TO THE ORBITING ELECTRONS. A SECOND ELECTROSTATIC FIELD   IS INDEPENDENTLY ESTABLISHED BETWEEN THE GRID AND A CONCENTRIC TITANIUM ROD ANODE AT AN OPTIMUM INTENSITY FOR ACCELERATING SPENT ELECTRONS THROUGH THE GRID TO BOMBARD AND SUBLIMATE THE TITANIUM FOR BURYING IONS AND COMBINING WITH ACTIVE GASES.

United States Patent [72] lnventors Mario Rablnowitz Menlo Park; EdwardL. Garwin, Los Altos Hills, Calif. [21] Appl. No. 811,028 [22] FiledMar, 27, 1969 [45] Patented June 28, 1971 [73] Assignee The UnitedStates of America asrepresented by the United States Atomic EnergyCommission [54] METHOD OF OPERATING AN ION-GETTER VACUUM PUMP WITH GUNAND GRID STRUCTURE ARRANGED FOR OPTIMUM lONIZATlON AND SUBLIMATION 8Claims, 3 Drawing Figs.

[52] U.S.Cl .I 315/108, 313/7, 313/237 [51] 1nt.Cl.., H0lj 7/16, H01 j17/22 [50] Field of Search .2 3l3/7,237; 315/108 [56] References CitedUNITED STATES PATENTS 2,292,087 8/1942 Ramo 3l3/237X 2,, Q E Q Q Q I0\g3a ADJUSTABLE Q woos VOLTAGE Q Primary Examiner.1ohn Kominski AssistantExaminer-Palmer C. Demeo Attorney-Roland A. Anderson ABSTRACT: Electronsinjected between a cylindrical housing and a coaxial cylindrical grid ata predetermined angular momentum spiral over long paths around the gridfor optimum ionization of gas molecules. A first electrostatic field isestablished between the housing and grid for accelerating ions to thehousing and for establishing an optimum kinetic energy of the orbitingelectrons. A second electrostatic field is independently establishedbetween the grid and a concentric titanium rod anode at an optimumintensity for accelerating spent electrons through the grid to bombardand sublimate the titanium for burying ions and combining with activegases.

Fl LAMENT VOLTAGE VARIABLE VOLTAGE SOURCE VARIABLE VOLTAGE souncePATENTEU JUN28 nan ADJUSTABLE ANODE VOLTAGE FILAMENT VOLTAGE VARIABLEVOLTAGE SOURCE VARIABLE VOLTAGE SOURCE INVENTORS MA RIO RAB/NOWITZEDWARD L. GARWIN ATTORNEY METHOD OF OPERATING AN ION-GETTER VACUUM PUMPWITH GUN AND GRID STRUCTURE ARRANGED FOR OPTIMUM IONIZATION ANDSUBLIMATION BACKGROUND OF THE INVENTION The invention relates toion-getter vacuum pumps, and more particularly, it relates togrid-controlled ion-getter vacuum pumps.

Ion-getter vacuum pumps are operated by introducing electrons into thepump at an energy level that creates ions upon collision with gasmolecules. The gas ions thus produced are accelerated by electric fieldsin the pump to impinge and penetrate into receptive pump surfaces.Simultaneously getter material (e.g., Ti, Ta) is sublimated or otherwisedeposited on these surfaces so that the buried ions are further covered.The fresh deposit of getter material also reacts with chemically activegases (e.g., O, and N without requiring ion formation. Thus, anion-getter pump is operated by removing molecules from the gaseous stateand putting them into the solid state.

One way of continuously supplying gettering material is to bombard thematerial to its sublimation temperature with the electrons that are usedto ionize the gas. This has the advantage that the getter material maybe thermally isolated easily as compared to resistive heating of thegettering material with its resultant thermal outgassing of electricalleads and nearby parts such as insulators and supporting structures.However, in known arrangements in which the electron bombardment methodof sublimation is used, a conflict arises as to the strength of electricfield to be used in the pump. A relatively low electrostatic field isrequired to give the injected electrons long paths before they areintercepted at an electrode. Long paths increase the probability ofelectron collisions with the gas molecules. A low field also permits theorbiting electrons to have an average kinetic energy corresponding tothe maximum ionization cross section of the gas, thereby increasing theprobability that the electrons will ionize the gas molecules when theycollide. In conflict with the advantages of a low electrostatic field, ahigh field is required to impart sufficient energy to the electrons toheat the gettering material to its sublimation temperature. However, ahigh electrostatic field tends to reduce the amount of ionization byshortening the electron paths and imparting a much higher than optimumaverage kinetic energy to the electrons.

Another problem found in filament electron source type of ion-getterpumps is the need to periodically replace the filament withoutintroduction of gas into the pump or vacuum system.

SUMMARY OF THE INVENTION In brief, the present invention pertains to anion-getter vacuum pump having a housing into which electrons areinjected at a circumferential point to travel long orbital spiralingpaths with optimum average kinetic energy for ionizing gas moleculeswithin the housing. Upon losing sufficient angular momentum, such as bycollision with gas molecules, the electrons are accelerated through agrid to impinge upon a centrally located anode which has getter materialmounted upon it. The major part of the electron paths is under theinfluence of a relatively low electrostatic field between the grid andthe pump housing. Upon reaching and passing through the grid, theelectrons come under the influence of a relatively high electrostaticfield between the grid and central anode that imparts sufficient energyto the electrons to raise the gettering material to its sublimationtemperature upon striking it, thereby causing the material to sublimeupon the inner surface of the pump housing for removal of gas moleculeswithin the housing.

Since the two electrostatic fields influence the electronsindependently, the fields may be adjusted separately to give optimumresults in their respective areas without adversely influencing theother area. Therefore, the field between the pump housing and the gridmay be independently adjusted for maximum ionization of the gasmolecules, while the field between the grid and central anode may beadjusted independently to provide the optimum sublimation rate.

Although various methods ofelectron injection may be used in conjunctionwith the aforementioned grid structure, we disclose a unique externalelectron gun which may be isolated from the housing by means of a valve.Thus the filament can easily be replaced without exposing the pump orvacuum system to external gas. This'increases the useful lifetime ofsuch a pump. With a small, relatively inexpensive valve, the electrongun can be isolated to change filaments, allowing the main body of thepump and the rest of the vacuum system to remain evacuated. Otherwise,the whole vacuum system would be exposed to atmospheric pressure, or avery large expensive valve would be required to isolate the system fromthe pump during filament replacement. Such an arrangement is impracticalnot only because of the expense and space consumption of a large valve,but also because it is undesirable to expose the pump to atmosphericpressure.

It is an object of the invention to ionize gases with maximum efficiencyin an ion-getter vacuum pump with a stream of electrons and to alsoindependently control the sublimation temperature of gettering materialwith the same electron stream.

Another object is to decouple accelerating fields of differing strengthsin an ion-getter vacuum pump.

Another object is to independently adjust the accelerating forces on astream of electrons during traversal by the electrons of an ionizationregion and subsequently a sublimation region in an iongetter vacuumpump.

Another object is to simultaneously produce electrostatic fields thatresult in independent optimum ionization and sublimation within anion-getter vacuum pump.

Another object is to efficiently and inexpensively replace the filamentof an ion-getter pump without affecting the vacuum within the pump.

Another object is to increase the useful operating lifetime of anion-getter pump.

Other objects and advantageous features of the invention will beapparent in a description of specific embodiments thereof, given by wayof example only, to enable one skilled in the art to readily practicethe invention, and described hereinafter with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of aniongetter vacuum pump incorporating the novel grid structure andelectron gun according to the invention.

FIG. 2 is a cross-sectional view of the ion-getter vacuum pump of FIG. 1taken along lines 2-2.

FIG. 3 is a cross-sectional view of a portion of a vacuum pump havingtwo internal hot filament cathodes as electron sources.

DESCRIPTION OF AN EMBODIMENT Referring to the drawing for a descriptionof an embodiment of the invention, there is shown in FIG. 1 a crosssection of an ion-getter vacuum pump 10, comprising a vacuum-tightcylindrical housing 12. An end 11 of the housing 12 is open forcommunication with a vacuum chamber (not shown) from which gas is to bepumped. The open end I1 is provided with a flange I4 for suitableconnection to the vacuum chamber.

Electrons are injected into the interior space of the cylindricalhousing 12 by means of an electron gun 31 which is mounted in one end ofthe housing. The gun 31 is angled with respect to the central axis ofthe housing so that the electrons are injected in a tangential directionthat has an axial component. This is desirable for efficiently directinga large number of electrons in long spiraling paths. However, the axialcomponent is not necessary as more fully discussed hereinafter. Arepresentative angle of the gun with respect to the axis of the housingis indicated in FIG. 1, whereas the tangency of the gun with respect tothe housing is indicated in FIG. 2. The electron gun 31 is comprised ofa hot filament cathode 33 connected to a voltage source 36 and isremovably mounted in a gun housing 34. A focusing cup 35 is negativelybiased and focuses electrons emitted from the filament 33. Anaccelerating anode 37 having a central aperture is mounted at theinjection end of the gun and is connected to an adjustable voltagesource 38 to inject a proper electron current into the pump. Theelectrons are injected by the gun into the housing between a cylindricalgrid 22 and the housing 12. An electrostatic field is maintained betweenthe housing and grid by means ofa variable DC voltage source 28 with thegrid biased positively with respect to the housing. At each end of thehousing, the field contains axial components directed toward me oppositeend. Electrons moving into an end of the housing are thereby given animpetus in the axial direction toward the opposite end, causing them tospiral back and forth around the grid 22 along orbital spiraling pathssuch as path 32. The electrons therefore spiral continuously around thegrid until angular momentum is lost by field asymmetries or by collisionwith gas molecules.

A grid 16 may be mounted across the end ill in electrical connectionwith the housing 112. The grid 116 permits movement of neutral gasparticles from the vacuum chamber to the interior of the pump housing.Since the grid 16 presents a surface that is electrically continuouswith the pump housing it produces a reflection of the electrons the sameas that produced as they approach the housing. By various methods, suchas adjusting the filament bias, the electrons may be injected with atotal energy that is intermediate the potential energy of the housingand the cylindrical grid 22. At this intermediate energy level theelectrons will not have sufficient energy to reach the housing or escapefrom the pump field even in the absence of the grid 16. In the absenceof grid 116, the excursion of an electron will be further up before itreaches an equipotential where all of its axial kinetic energy isconverted into potential energy; whereupon it will be reflected backdown into the pump.

An anode electrode 18 is mounted along the central axis of the housing12 but is electrically isolated therefrom by means of an insulatingbushing R9. The anode 18 includes gettering material in the form ofcylindrical slugs 20 of titanium, for example, that are suitablyattached to the electrode along its length. The cylindrical grid 22 ismounted concentrically with the housing l2 and electrode 18 on aninsulating mounting 24, which electrically isolates the grid from therest of the pump. A shadow shield 44 prevents deposition of getteringmaterial on bushing 19. Another shadow shield 25 surrounds theinsulating mounting 24 to prevent deposit of vaporized getteringmaterial on the mounting. A lead 26 extends through an insulatingbushing 23 in the pump housing and connects the grid 22 to the positiveterminal of the variable DC voltage source 28, which has its negativeterminal connected to the pump housing 12. A cylindrical shield 27serves both to shield the insulators l9 and 23 from vaporized getteringmaterial as well as to support the insulating mounting 24. A secondadjustable DC voltage source 29 has its positive terminal connected tothe lower end of the anode electrode 18 and its negative terminalconnected to the pump housing.

In operation of the pump it), a first electrostatic field with radialand axial components and essentially no azimuthal components isestablished between the cylindrical grid 22 and the cylindrical housingll2 with the voltage source 28. Electrons are injected into the pumpwith sufficient initial angular momentum that they cannot fall into asmall enough radius to be captured at the grid 22. The angular momentumof electrons circulating around the grid is conserved as the field canapply no torque on the electrons about the pump axis. The electronscontinue to orbit until they lose angular momentum by gas collisions orby perturbations of the cylindrical symmetry of the field. The axialcomponents of the field near the ends of the pump serve to reflect theelectrons back toward the center of the pump. Thus the electrons willtraverse spirallike paths around the grid from one end of the pump tothe other and back.

As electrons lose angular momentum, the majority become spent andfinally fall in through the grid 22 and come under the influence of arelatively high electrostatic field between the grid 22 and anode 18 asestablished with the DC voltage source 29. The high field acceleratesthe electrons causing them to impinge upon and heat the getteringmaterial 20 on the anode 18. By adjusting the accelerating voltage 29,the sublimation rate can be independently set to a desired value.

To maximize ion production, it is necessary that the average kineticenergy of the electrons be at that value corresponding to the maximumionization rate of the inert gases present. The ionization cross sectionof gases starts at some threshold value of electron kinetic energy,rises quickly to a maximum value, and then decreases slowly withincreasing electron kinetic energy. Therefore, if the average electronkinetic energy is more or less than the value corresponding to themaximum ionization cross section, then the average cross section may beconsiderably below the maximum. ln ion-getter pumps of prior art inwhich the same electrons perform the dual functions of producingionization and then sublimation, the average electron kinetic energy isconsiderably higher than corresponds to the maximum ionization crosssection due to the need for a high voltage on the anode to producesublimation temperatures. Thus, the ionization rate in those pumps isappreciably less than the maximum value possible. If the anode voltagein the prior art pumps were reduced to give the electrons the optimumaverage kinetic energy, the reduced sublimation rate would decrease theactive gas pumping speed due to the reduced amount of gettering materialavailable for combination with the active gas; in addition, the inertgas pumping speed would be reduced, even though the ionization ratewould be increased, since the inert gases require both ionization andburial by the sublimed gettering material for permanent removal.

As taught in the present invention, the cylindrical grid 22 is held atthe proper voltage V,, to give the electrons the op timum averagekinetic energy which corresponds to the maximum ionization rate. Theproper grid voltage may be determined experimentally. In practice, theremay be more than one inert gas present in the pump such as, for example,helium, argon, and methane. it may be desired to optimize the pump withrespect to each gas separately in some sequential order, and this mayalso be done. To act as a guideline in vary ing the various parameters,one may use the following approximate equation:

V-.=2 r C where V is the voltage on the grid 22, R is the radius of thepump housing 12, r is the radius of the grid 22, T is the desiredaverage value of electron kinetic energy, E is the total electronenergy, and e is the electronic charge.

The anode 18 is held at whatever variable voltage is required to producethe desired sublimation rate. By the addition of the grid 22, the fieldsbetween the housing and grid, and grid and anode are decoupled. Theprocess of sublimation and its requirements are thereby separated fromthe orbiting requirements for maximum ionization even though the sameelectrons still perform the dual tasks of ionization and sublimation. Inaddition, the use of separate fields permits a large quantity ofgettering material to be mounted on the anode. In the prior art, withonly a single field, a large amount of getter ing material results in alarge diameter anode which decreases the path length of the orbitingelectrons. A large amount of gettering material also requires more powerto be heated to its sublimation temperature due to its large radiatingarea. The high power requires a high field which further shortens theelectron paths in the prior art. Thus, in the present invention, withthe requirements for long electron paths and sublimation separated, alarge amount of gettering material may be mounted on the anode toincrease the operating lifetime of the pump without adversely affectingthe ion pumping speed.

In order to permit replacement of the hot filament cathode 33 in theelectron gun 31, a straight-through valve 41 is mounted in the gunhousing 34. The valve 41 is provided with a passage 40 which is axiallyaligned with the central axis of the gun to pennit passage of thecathode 33, the focusing cup 35, and the anode 37 to be positioned atany desired distance relative to an entrance aperture 39 of the pump asdetermined by a positioning bellows 46. The cathode 33, focusing cup 35,and anode 37 are mounted on an insulating bushing 47 which is attachedto the bellows 46 with electrical leads extending through the bushingfor connection to respective voltage sources. In the event of theburnout of the filament 33, the valve 41 may he closed to seal the pumphousing by retracting the bellows 46, and sealing the head 48 of thevalve over the passage 40. The cathode may then be removed and replacedwithout affecting the vacuum within the pump. This arrangement alsopermits adjustment of the spacing between cathode 33 and anode 37. Uponreplacement of the cathode 33, the gun space may be evacuated through aport 42 prior to the opening of the valve 4| and extension of thebellows 46. The valve 42 may be either a very small valve or a pinch-offtube. Thus the useful operating lifetime of the pump is furtherincreased.

Tubing 45 is provided for circulation of a coolant for maintaining thepump housing 12 at a temperature substantially below the level at whichthermal desorption of gas interferes with the pump's operation. The pumphousing is thereby eliminated as a source of gas.

Although, as discussed hereinbefore, electrons may efficiently bedirected in long spiraling paths with an electron gun mounted in adirection having an axial component and although it is convenient tomount the gun external to the housing to enable isolation of the gun,satisfactory operation of an ion pump according to the invention may beobtained with electron guns such as shown in FIG. 3 wherein two hotfilament cathodes 50 are mounted and sealed in the lower end of the pumphousing 12 and are biased at a level that is negative with respect tothe space potential that would otherwise be present at the filamentposition and positive with respect to the housing 10. The electrons arestill made to move in spiraling paths since the cathodes 50 are locatednear the end of the housing where the field between the grid and housinghas an axial component of direction which gives each electron an impetusin the axial direction. Alternatively, the electron injection means ofFIG. 1 can be made to operate according to the same principle by whichthe electrons are injected into the pump shown in FIG. 3. This may bedone by eliminating the anode 37 and focusing cup 35 and inserting thecathode 33 to a point inside the housing 12 where the positive potentialin the first electrostatic field acts as a virtual anode to draw theelectrons from the cathode 33.

We claim:

1. A method for removing gas molecules from a space, comprising:

the steps of injecting electrons into an annular volume of said space,between a cylindrical grid and a coaxial cylindrical cathode to collidewith and ionize the gas molecules;

applying a first voltage V, between the grid and cathode at a level thatestablishes a first electrostatic field between the grid and cathodethat gives the injected electrons an average kinetic energy thatcorresponds to the maximum ionization cross section of the gasmolecules, said first voltage being applied in a direction that causesthe electrons to be attracted to the grid, said first voltage, V,, beingdefined by the relationship:

R is the radius of the cathode, r is the radius of the grid, T is theaverage kinetic energy in electron volts which corresponds to themaximum ionization cross section of the gas molecules, E is the totalelectron energy in electron volts, and e is the unit electronic charge;and

applying a second voltage between the grid and an anode that is coaxialwith the grid and cathode, with the grid interposed between the anodeand cathode, said second voltage being higher than said first voltage,thereby establishing a second electrostatic field for accelerating theinjected electrons towards the anode to bombard gettering materialattached to the anode and thereby cause the material to sublime on thecathode.

2. The method of claim I, wherein said gas molecules are constituentsofa mixture ofa plurality of individual gases, and further including thesteps of applying successive voltages between the grid and cathode toestablish the intensity of the first field to successively correspond tothe maximum ionization cross sections of the individual gases of saidplurality of gases.

3. The method of claim 1, wherein successive different gases are exposedto said annular volume, further including the steps of applyingsuccessive voltages between the grid and cathode to establish theintensity of the first field to successively correspond to the maximumionization cross sections of the successive different gases exposed tosaid annular volume.

4. The method of claim 1, further including the step of adjusting theinitial kinetic energy of the injected electrons.

5. The method of claim 1, further including the steps of adjusting thefirst electrostatic field to have a potential gradient that is lowerthan the potential gradient of the second electrostatic field; andadjusting the second field for optimum sublimation of the getteringmaterial.

6. The method of claim 1, wherein the first electrostatic field isestablished to have a cylindrical symmetry about the grid, the firstfield having an elongated radial central zone for attracting electronsto the grid and ionized gas molecules to the cathode, the first fieldhaving end regions adjacent the central zone, said end regions havingradial and axial components which exert a force on the electrons towardthe central zone.

7. The method of claim 1, wherein said electrons are injectedtangentially and in a partially axial direction into the annular volume.

8. The method of claim 1, wherein said electrons are injected from asource that is external to the annular volume.

